1. Field of the Invention
The present invention relates to improvements in a reinforced concrete structure employing a tapered slab, which is formed of a two-dimensional continuous array of upside-down rectangular pyramids with their apexes placed at column centers and their bottom faces placed along an upper surface of a floor slab.
2. Description of the Prior Art
The above-referenced tapered slab was developed to be a replacement for the conventional flat slab as a novel slab adapted for a large span structure and was described in the copending Japanese Utility Model Application No. 55-110229 filed on Aug. 5, 1980. Here, at first, brief description will be made of a structure employing the tapered slab on which this invention is based.
In the proposed tapered slab structure, since the slab is formed of a two-dimensional continuous array of upside-down rectangular pyramids with their apexes placed at column centers and their bottom faces placed along an upper surface of a floor slab and capitals as well as drop panels used in the conventional flat slab structure are eliminated, form work can be completed by merely assembling four large-sized trapezoidal form boards around each column. Hence, form work can be greatly simplified, work for preparing form board can be minimized, a number of reuses of form lumbers is increased, and thus the tapered slab structure is favorable for saving resources.
In the case of considering from a mechanical point of view, the floor slab has such vertical cross-section that it is thinnest at a midpoint between columns and thickest at the opposite ends close to the columns. On the other hand, bending and shearing stresses in the floor slab are concentrated at the end portions close to the columns and become very small at a midpoint between the columns. Consequently, the floor slab consisting of a tapered slab is thick at the end portions close to the columns where stresses are large, and thin at the midpoint between the columns where stresses are small. Hence, the cross-section of the floor slab has a reasonable configuration in view of a cross-sectional efficiency.
Moreover, since the upper surface of the floor slab is a horizontal plane and the lower surface has a pyramid-shaped surface portion with its apex located at a midpoint between columns, the floor slab forms, as a whole, a two way arch, so that a large part of a load is transmitted in the form of axial forces along the surface of the arch, resulting a large reduction of a bending stress in the floor slab, and consequently the amount of reinforcements to be used for the floor slab can be reduced. The above-mentioned arch action has the mechanically most remarkable advantages of the present invention.
Furthermore, the structure employing a tapered slab is deemed to have a cross-sectional shape which is formed by scraping out a lower portion in the middle between columns of a floor slab in the conventional flat slab structure, and so, as compared to the conventional flat slab structure, the dead load is reduced. Hence, bending and shearing stresses of a floor slab are reduced, and this reduction contributes, jointly with the aforementioned arch action, to the saving of the amount of reinforcements.
Still further, in the tapered slab structure, since unevenness normally formed by girders and beams is not present on the lower surface of the floor slab, in the case of utilizing the space between the floor slab and a ceiling extended thereunder as a space for air conditioning, there is an advantage that a flow of air therethrough is smooth and hence an air conditioning efficiency can be enhanced. In addition, even if the level of the ceiling is raised up to the lowest points, that is, the apexes of the lower surface of the floor slab, owing to the existence of the upside-down rectangular pyramids, pyramid-shaped spaces are always left between the ceiling and the floor slab and these spaces can be effectively utilized as ducts. Therefore, a floor height of a building can be reduced and consequently the structural materials can be saved.
As described above, the tapered slab structure has largely contributed to saving the amount of building materials owing to the inherent structural characteristics of the tapered slab. However, in order to effectively utilize the inherent structural characteristics, a reaction force acting upon an arch support for maintaining an arch action must be well controlled.
Although the reaction forces for the respective spans are offset with each other at an inner arch support in a multi-span building because the horizontal components of the reaction forces for the adjacent spans at the inner arch support have the same magnitude and the opposite directions, the reaction forces for the outermost spans in a multi-span building or the reaction forces for the span in a single-span building are directly transmitted to columns, and so, the arch action is eliminated.